The present invention relates generally to non-contact tracking of objects using a magnetic field, and specifically to counteracting the effect of an intruding field-responsive article in the field.
Non-contact electromagnetic tracking systems are well known in the art, with a wide range of applications.
U.S. Pat. No. 5,391,199, to Ben-Haim, which is assigned to the assignee of the present application and whose disclosure is incorporated herein by reference, describes a system for generating three-dimensional location information regarding a medical probe or catheter. A sensor coil is placed in the catheter and generates signals in response to externally applied magnetic fields. The magnetic fields are generated by three radiator coils, fixed to an external reference frame in known, mutually spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils.
PCT patent publication WO/96/05768, filed Jan. 24, 1995, which is assigned to the assignee of the present application and whose disclosure is incorporated herein by reference, describes a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates. As in the case of the ""539 patent application described above, the radiator coils preferably operate simultaneously at different frequencies, for example at 1000, 2000 and 3000 Hz, respectively.
The above tracking systems rely on separation of position-responsive signals into components, most typically frequency components, wherein each such component is assumed to correspond uniquely to a single radiator coil, in a known position, radiating a magnetic field having a regular, well-defined spatial distribution. In practice, however, when a metal or other magnetically-responsive article is brought into the vicinity of the catheter or other object being tracked, the magnetic fields generated in this vicinity by the radiator coils are distorted. For example, the radiator coil""s magnetic field may generate eddy currents in such an article, and the eddy currents will then cause a parasitic magnetic field to be radiated. Such parasitic fields and other types of distortion can lead to errors in determining the position of the object being tracked.
U.S. Pat. No. 5,767,669 to Hansen et al., whose disclosure is incorporated herein by reference, describes a method for subtracting eddy current distortions produced in a magnetic tracking system. The system utilizes pulsed magnetic fields from a plurality of generators, and the presence of eddy currents is detected by measuring rates of change of currents generated in sensor coils used for tracking. The eddy currents are compensated for by adjusting the duration of the magnetic pulses.
U.S. Pat. No. 4,945,305 to Blood, whose disclosure is incorporated herein by reference, describes a tracking system which avoids the problems of eddy currents by using pulsed DC magnetic fields. Sensors which are able to detect DC fields are used in the system, and eddy currents are detected and adjusted for by utilizing the decay characteristics and the amplitudes of the eddy currents.
European Patent Application EP 0964261A2, to Dumoulin, whose disclosure is incorporated herein by reference, describes systems for compensating for eddy currents in a tracking system using alternating magnetic field generators. In a first system the eddy currents are compensated for by first calibrating the system free from eddy currents, and then modifying the fields generated when the eddy currents are detected. In a second system the eddy currents are nullified by using one or more shielding coils placed near the generators.
FIG. 1 is a graph showing a relation of the permeability xcexc of a ferromagnetic material in a magnetic field vs. frequency f at which the field is being generated, as is known in the art. Permeability xcexc is a factor in the phase shift generated by the magnetic field. The graph applies to a change of the permeability xcexc of the ferromagnetic material, generated for an article wherein eddy currents are formed. The change reflects the phase shift in a sensor, caused by the article, vs. the frequency f As. is known in the art, additional factors affecting the phase shift are geometry of the article, and conductivity of the material. The graph shows a virtually linear change in permeability for small changes in frequency.
It is an object of some aspects of the present invention to provide methods and apparatus for non-contact tracking of an object in an energy field in the presence of an article which interferes with the field.
It is another object of some aspects of the present invention to provide methods and apparatus for minimizing the effect of an article which interferes with an energy field used for non-contact tracking of an object.
In a preferred embodiment of the present invention, an object tracking system comprises one or more sensor coils adjacent to a locatable point on an object being tracked, and one or more radiator coils, which generate alternating energy fields comprising magnetic fields, in a vicinity of the object when driven by respective alternating electrical currents. For each radiator coil, a frequency of its alternating electrical current is scanned through a plurality of values so that, at any specific time, each of the radiator coils radiates at a frequency which is different from the frequencies with which the other radiator coils are radiating.
The sensor coils generate electrical signals responsive to the magnetic fields, which signals are received by signal processing circuitry and analyzed by a computer or other processor. When a metal or other field-responsive article is in the vicinity of the object, the signals typically include position signal components responsive to the magnetic fields generated by the radiator coils at their respective instantaneous driving frequencies, and parasitic signal components responsive to parasitic magnetic fields generated due to the article. The parasitic components are typically equal in frequency to the instantaneous frequency of the driving frequency, but are shifted in phase, so that the effect at each sensor coil is to produce a combined signal having a phase and an amplitude which are shifted relative to the signal when no field-responsive article is present. The phase-shift is a function of the driving frequency, and so will vary as each driving frequency is scanned. The computer processes the combined signal to find which frequency produces a minimum phase-shift, and thus a minimum effect of the parasitic components, and this frequency is used to calculate the position of the object. Varying the driving frequency until the phase shift is a minimum is an effective method, not known in the art, for reducing the effect of field-responsive articles on the signal.
In preferred embodiments of the present invention, an alternative method is also used in order to find a value of the position signal, i.e., of the signal produced without interfering effects of the field-responsive article. Measurements of the value of the combined signal are made at a plurality of frequencies. The values obtained are used to solve a plurality of simultaneous equations comprising the position signal as one of the unknowns in the equations. Thus, varying the driving frequency enables the position signal to be determined in the presence of interfering signals from field-responsive articles.
The present invention relies on the fact that parasitic magnetic fields, generated by metal or other field-responsive articles that receive and re-radiate energy from a radiator coil magnetic field are typically at the same frequency as the radiator coil field, but are shifted in phase relative thereto. The phase shift and the amplitudes of the parasitic fields generally depend on properties of the article, including dielectric constant, magnetic permeability and geometrical shape.
However, both the phase shift and the amplitude of the parasitic fields can be assumed to be linearly dependent on the value of the frequency generating the parasitic field.
There is therefore provided, according to a preferred embodiment of the present invention, a method for tracking an object including:
producing an unperturbed energy field at a plurality of predetermined frequencies in the vicinity of the object;
determining a characteristic of a perturbing energy field induced responsive to the unperturbed field, due to introduction of an article responsive to the unperturbed field into the vicinity of the object;
receiving a plurality of resultant signals responsive to the unperturbed and perturbing energy fields generated at a location of the object after introduction of the article;
determining an optimal frequency for the unperturbed energy field from amongst the plurality of predetermined frequencies responsive to a parameter of the resultant signals; and
determining spatial coordinates of the object responsive to the resultant signal at the optimal frequency.
Preferably, producing the unperturbed energy field at the plurality of predetermined frequencies includes scanning the frequencies sequentially.
Further preferably, producing the unperturbed energy field at the plurality of predetermined frequencies includes multiplexing at least some of the frequencies.
Preferably, receiving the plurality of resultant signals includes:
measuring a baseline phase value xcfx86107  of each of the plurality of resultant signals at the respective plurality of predetermined frequencies before introduction of the article; and
measuring a phase shift xcfx86xcfx89total at the respective plurality of predetermined frequencies after introduction of the article, so that the parameter comprises a term |xcfx86xcfx89totalxe2x88x92xcfx86xcfx89| for each of the plurality of predetermined frequencies; and
wherein determining the optimal frequency includes determining a frequency xcfx89 at which |xcfx86xcfx89totalxe2x88x92xcfx86xcfx89| is a minimum.
Preferably, determining spatial coordinates of the object includes determining spatial coordinates responsive to an amplitude of a signal |Mxcfx89| at the frequency xcfx89.
Further preferably, determining spatial coordinates of the object includes determining spatial coordinates responsive to a phase of a signal Mxcfx89 at the frequency xcfx89.
Preferably, producing the energy fields includes producing magnetic fields. Preferably, receiving the signals includes receiving electrical signals which are generated responsive to the magnetic fields.
There is further provided, according to a preferred embodiment of the present invention, a method for tracking an object, including:
producing an unperturbed energy field comprising a plurality of predetermined frequencies in the vicinity of the object;
producing a perturbing energy field by introduction of an article responsive to the unperturbed field into the vicinity of the object;
receiving a respective plurality of signals responsive to the unperturbed and perturbing energy fields generated at a location of the object after introduction of the article; and
determining one or more factors conditional on spatial coordinates of the object responsive to the plurality of signals and the respective frequencies.
Preferably, determining the one or more factors includes:
assuming a phasor {overscore (A)}xcfx89 of a signal responsive to the unperturbed energy field and a phasor {overscore (A)}xe2x80x2xcfx89 of a signal responsive to the perturbing energy field to be directly proportional to a plurality of predetermined currents generating the fields; and
assuming a phase xcfx86xcfx89 of the signal responsive to the unperturbed energy field and a phase xcfx86xcfx89xe2x80x2 of the signal responsive to the perturbing energy field to be linearly dependent on the plurality of predetermined frequencies.
Preferably, the plurality of frequencies includes at least four frequencies, and the one or more factors include the spatial coordinates of the object.
Preferably, receiving the plurality of signals comprises receiving at least four values of a signal Mi at the at least four frequencies, and determining the one or more factors includes:
determining a value of a position signal amplitude A0, generated responsive to the unperturbed energy field, by substituting respective values of the signal Mi into an equation
{overscore (M)}i={overscore (A)}i+aixe2x80x2eixcfx86ixe2x80x2
wherein {overscore (M)}i is a phasor representing a measured field, {overscore (A)}i is a phasor representing the unperturbed field, aixe2x80x2 represents an amplitude of the perturbing field, xcfx86ixe2x80x2 represents a phase of the perturbing field, and i represents at least four numbers respectively corresponding to the at least four frequencies, so as to generate at least four equations; and
solving the at least four equations for the position signal amplitude A0.
There is further provided, according to a preferred embodiment of the present invention, object tracking apparatus, comprising:
a radiator, which generates an energy field at a plurality of predetermined frequencies in the vicinity of the object;
a sensor, fixed to the object, which generates a plurality of signals responsive to the energy field and to an interfering article responsive to the energy field; and
signal processing circuitry, which receives the plurality of signals from the sensor and determines an optimal frequency for the energy field from amongst the plurality of predetermined frequencies responsive to a parameter of the signals, and which determines position coordinates of the object responsive to the signal at the optimal frequency.
Preferably, the radiator generates the energy field at the plurality of predetermined frequencies by scanning the frequencies sequentially.
Further preferably, the radiator generates the energy field at the plurality of predetermined frequencies by multiplexing at least some of the frequencies.
Preferably, the parameter includes a phase shift, and the optimal frequency includes the frequency where the phase shift is a minimum.
Preferably, the signal processing circuitry determines the position coordinates of the object responsive to an amplitude of one of the plurality of signals at the frequency where the phase shift is a minimum.
Preferably, the energy field includes a magnetic field.
Preferably, the plurality of signals include a plurality of electrical signals which are generated responsive to the magnetic field.
There is further provided, according to a preferred embodiment of the present invention, object tracking apparatus, including:
a radiator, which generates an energy field including a plurality of predetermined frequencies in the vicinity of the object;
a sensor, fixed to the object, which generates a respective plurality of signals responsive to the energy field and to an interfering article responsive to the energy field; and
signal processing circuitry, which receives the plurality of signals from the sensor and determines one or more factors conditional on spatial coordinates of the object responsive to the signals and their corresponding frequencies.
Preferably, the plurality of frequencies includes at least four frequencies, and wherein the one or more factors comprise the spatial coordinates of the object.
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings, in which: